1. Field of Invention
The present invention relates to a method for exfoliating a detached member, and in particular, a transferring method for exfoliating a transferred layer comprising a thin film such as a functional thin film and for transferring it onto a transfer member such as a transparent substrate. Also, the present invention relates to a transferring method of a thin film device, a thin film device, a thin film integrated circuit device, and a liquid crystal display device produced using the same.
2. Description of Related Art
Production of liquid crystal displays using thin film transistors (TFTs), for example, includes a step for forming thin film transistors on a transparent substrate by a CVD process or the like.
The thin film transistors are classified into those using amorphous silicon (a-Si) and those using polycrystalline silicon (p-Si), and those using polycrystalline silicon are classified into those formed by a high temperature process and those formed by a low temperature process.
Since the formation of such thin film transistors on a substrate involves treatment at a relatively high temperature, a heat resistant material, that is, a material having a high softening point and a high melting point must be used as the transparent substrate. At present, in the production of TFTs by high temperature processes, transparent substrates composed of quartz glass which are sufficiently resistive to a temperature of approximately 1,000xc2x0 C. are used. When TFTs are produced by low temperature processes, the maximum process temperature is near 500xc2x0 C., hence heat-resisting glass which is resistive to a temperature near 500xc2x0 C. is used.
As described above, a substrate for use in forming thin film devices must satisfy the conditions for producing these thin film devices. The above-mentioned xe2x80x9csubstratexe2x80x9d is, however, not always preferable in view of only the steps after fabrication of the substrate provided with thin film devices is completed.
For example, in the production process with high temperature treatment, quartz glass or heat-resisting glass is used, however, they are rare and very expensive materials, and a large transparent substrate can barely be produced from the material.
Further, quartz glass and heat-resisting glass are fragile, easily broken, and heavy. These are severe disadvantages when a substrate provided with thin film devices such as TFTs is mounted into electronic units. There is a gap between restriction due to process conditions and preferred characteristics required for products, hence it is significantly difficult to satisfy both the restriction and characteristics.
The present invention has been achieved in view of such a problem, and has an object to provide an exfoliating method, which permits easy exfoliation regardless of characteristics of the detached member and conditions for exfoliating, and transferring to various transfer members. Another object is to provide a novel technology which is capable of independently selecting a substrate used in production of thin film devices and a substrate used when the product is used (a substrate having preferable properties for use of the product). A further object is to provide a novel technology not causing deterioration of characteristics of thin film devices which are transferred onto a substrate, by decreasing the optical energy radiated to the separable layer causing ablation in the transferring process.
1. First, a method for exfoliating a detached member or a transferred layer from a substrate for production is disclosed. The inventions are as follows:
(1) An exfoliating method in accordance with the present invention is a method for exfoliating a detached member, which is present on a substrate with a separation layer therebetween, from the substrate, wherein the separation layer is irradiated with incident light so as to cause exfoliation in the separation layer and/or at the interface, and to detach the detached member from the substrate.
(2) A method for exfoliating a detached member, which is present on a transparent substrate with a separation layer therebetween, from the substrate, wherein the separation layer is irradiated with incident light from the side of the substrate so as to cause exfoliation in the separation layer and/or at the interface, and to detach the detached member from the substrate.
(3) A method for exfoliating a transferred layer formed on a substrate with a separation layer therebetween from the substrate and transferring the transferred layer onto a transfer member, wherein after the transfer member is adhered to the opposite side of the transferred layer to the substrate, the separation layer is irradiated with incident light so as to cause exfoliation in the separation layer and/or at the interface, and to detach the transferred layer from the substrate to transfer onto the transfer member.
(4) A method for exfoliating a transferred layer formed on a transparent substrate with a separation layer therebetween from the substrate and transferring the transferred layer onto a transfer member, wherein after the transfer member is adhered to the opposite side of the transferred layer to the substrate, the separation layer is irradiated with incident light from the side of the substrate so as to cause exfoliation in the separation layer and/or at the interface, and to detach the transferred layer from the substrate to transfer onto the transfer member.
(5) An exfoliating method includes a step for forming a separation layer on a transparent substrate, a step for forming a transferred layer on the separation layer directly or with a given interlayer therebetween, a step for adhering the transfer member to the opposite side of the transferred layer to the substrate, and a step for irradiating the separation layer with incident light from the side of the substrate so as to cause exfoliation in the separation layer and/or at the interface, and to detach the transferred layer from the substrate to transfer onto the transfer member.
In connection with these inventions, the following inventions are disclosed.
After transferring the transferred layer onto the transfer member, a step for removing the separation layer adhering to the side of the substrate and/or transfer member may be provided.
A functional thin film or a thin film device may be used as the transferred layer. Particularly, a thin film transistor is preferably used as the transferred layer. Preferably, the transfer member is a transparent substrate.
When the maximum temperature in the formation of the transferred layer is Tmax, it is preferred that the transfer member be composed of a material having a glass transition point (Tg) or softening point which is lower than Tmax. Particularly, it is preferred that the transfer member be composed of a material having a glass transition point (Tg) or softening point which is lower than 800xc2x0 C.
It is preferable that the transfer member be composed of a synthetic resin or glass.
It is preferable that the substrate has thermal resistance. In particular, when the maximum temperature in the formation of the transferred layer is Tmax, it is preferred that the substrate be composed of a material having a distortion point which is lower than Tmax.
In the above-mentioned exfoliating methods, the exfoliation of the separation layer is caused by an elimination of or a decrease in the adhering force between atoms or molecules in the constituent substances in the separation layer.
It is preferable that the incident light be laser light. Preferably, the laser light has a wavelength of 100 nm to 350 nm. Alternatively, the laser light has a wavelength of 350 nm to 1,200 nm.
It is preferable that the separation layer is composed of amorphous silicon. Preferably, the amorphous silicon contains 2 atomic percent or more of hydrogen (H).
The separation layer may be composed of a ceramic. Alternatively, the separation layer may be composed of a metal. Alternatively, the separation layer may be composed of an organic polymer. In this case, it is preferable that the organic polymer has at least one adhere selected from the group consisting of xe2x80x94CH2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CONHxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94Nxe2x95x90Nxe2x80x94, and xe2x80x94CHxe2x95x90Nxe2x80x94. Further, it is preferable that the organic polymer has an aromatic hydrocarbon group in the chemical formula.
2. Next, inventions in which the above-mentioned separation layer includes a plurality of composites are disclosed. These inventions are as follows.
First, the separation layer in the inventions disclosed in paragraph 1 includes a composite with a plurality of layers. Further, the separation layer includes at least two layers having different compositions or characteristics.
It is preferable that the separation layer includes an optical absorption layer for absorbing the incident light and another layer having a different composition or property from the optical absorption layer. Preferably, the separation layer includes the optical absorption layer for absorbing the incident light and a shading layer for shading the incident light. Preferably, the shading layer lies at the opposite side of the optical absorption layer to the incident light. Preferably, the shading layer is a reflection layer for reflecting the incident light. Preferably, the reflection layer is composed of a metallic thin film.
3. A method for transferring a thin film device, which is used as a detached member or a transferred member, will now be disclosed.
A method for transferring a thin film device on a substrate onto a transferred member includes: a step for forming a separation layer on the substrate; a step for forming a transferred layer including the thin film device onto the separation layer; a step for adhering the transferred layer including the thin film device to the transfer member with an adhesive layer, a step for irradiating the separation layer with light so as to cause exfoliation in the separation layer and/or at the interface; and a step for detaching the substrate from the separation layer.
In accordance with the present invention, for example, a separation layer having optical absorption characteristics is provided on a substrate having high reliability in device production, and thin film devices such as TFTs and the like are formed on the substrate. Next, although not for limitation, the thin film devices are adhered to a given transfer member, for example, with an adhesive layer, so as to cause an exfoliation phenomenon in the separation layer, which results in a decrease in adhering between the separation layer and the substrate. The substrate is detached from the thin film devices by the force applied to the substrate. A given device with high reliability can be thereby transferred or formed onto any transfer members.
In the present invention, either the step for adhering the thin film devices (the transferred layer including the thin film devices) to the transfer member with the adhesive layer or the step for detaching the substrate from the thin film devices may precede. When handling of the thin film devices (the transferred layer including the thin film devices) after detaching the substrate is troublesome, however, it is preferable that the thin film devices be adhered to the transfer member, and then the substrate be detached.
When an adhesive layer for adhering the thin film devices to the transfer member is, for example, a substance having planation, the uneven face formed on the surface of the transferred layer including the thin film devices is negligible by the planation, the adhering to the transfer member is satisfactorily performed. The substrate may be a transparent substrate, and thus the separation layer is irradiated with the light through the transparent substrate. The use of, for example, a transparent substrate, e.g. a quartz substrate, permits production of thin film devices with high reliability and collective irradiation of the entire separation layer with the light from the rear side of the substrate, resulting in an improvement in the transfer efficiency.
4. Inventions in which parts of the steps, disclosed in the above-mentioned paragraph 3, in the method for transferring the thin film device will now be disclosed. These inventions are as follows:
(1) A method for transferring a transferred layer including a thin film device forming on a substrate onto a transfer member comprising: a first step for forming an amorphous silicon layer on the substrate; a second step for forming the transferred layer including the thin film device on the amorphous silicon layer; a third step for adhering the transferred layer including the thin film device to the transfer member with an adhesive layer; a fourth step for irradiating the amorphous silicon layer with light through the substrate so as to cause exfoliation in the amorphous silicon layer and/or at the interface and to decrease the adhering force between the substrate and the transferred layer; and a fifth step for detaching the substrate from the amorphous silicon layer; wherein the transferred layer formed in the second step includes a thin film transistor, and the thickness of the amorphous silicon layer formed in the first step is smaller than the thickness of the channel layer of the thin film transistor formed in the second step.
In this invention, the amorphous silicon layer is used as the layer formed on the substrate in the first step and causes exfoliation by light irradiation. In the amorphous silicon layer as shown in FIG. 39, optical energy, which is radiated in the amorphous silicon layer and which is required for exfoliation (referred to as ablation in FIG. 39), decreases as the thickness decreases.
The transferred layer formed in the second step includes the thin film transistor as a thin film device, its channel layer is formed of silicon, e.g., polycrystalline silicon or amorphous silicon, and the transferred layer has a thickness of more than 25 nm, for example, approximately 50 nm. In this invention, the thickness of the amorphous silicon as the separation layer (ablation layer) formed in the first step is smaller than that of the channel layer of the thin film transistor in the transferred layer. The energy consumed in the light irradiation step therefore decreases, and the light source can be miniaturized. Further, since optical energy by irradiation is low, the deterioration of the thin film device is suppressed if the light leaked from the amorphous silicon layer is incident on the thin film device.
Now, the thickness of the amorphous silicon layer is set to 25 nm or less. As described above, the optical energy, which is radiated in the amorphous silicon layer and which is required for exfoliation, decreases as the thickness decreases, hence the optical energy is significantly low at this thickness. It is preferable that the thickness of the amorphous silicon layer be in a range from 5 nm to 25 nm, more preferably 15 nm or less, and most preferably 11 nm or less in order to further decrease the optical energy, which is radiated in the amorphous silicon layer and which is required for exfoliation.
In the second step, the amorphous silicon layer is formed by a low pressure chemical vapor deposition (LPCVD) process. The amorphous silicon layer formed by the LPCVD process has a higher adhesion compared with a plasma CVD process, an atmospheric pressure (AP) CVD process, or an ECR process, hence there is not much risk of failures, such as evolution of hydrogen and flaking of the film, during the formation of the transferred layer including the thin film device.
(2) A method for transferring a transferred layer including a thin film device on a substrate onto a transfer member comprising: a step for forming a separation layer onto the substrate; a step for forming a silicon-based optical absorption layer on the separation layer; a step for forming the transferred layer including the thin film device on the silicon-based optical absorption layer; a step for adhering the transferred layer including the thin film device to the transfer member with an adhesive layer; a step for irradiating the separation layer with light through the substrate so as to cause exfoliation in the separation layer and/or at the interface; and a step for detaching the substrate from the separation layer.
In accordance with this invention, if light leaks from the separation layer, the leaked light is absorbed in the silicon-based optical absorption layer before it is incident on the thin film device. No light is therefore incident on the thin film device, hence the thin film device is prevented from characteristic deterioration due to the incident light. The transferred layer including the thin film device can be formed on the silicon-based optical absorption layer. Metallic contamination will therefore not occur as in the case forming the transferred layer onto a metallic layer reflecting light, and the thin film device can be formed by an established thin film deposition technology.
The separation layer and the optical absorption layer are formed of amorphous silicon, and a step for providing a silicon-based intervening layer between the separation layer and the optical absorption layer. As shown in FIG. 39, the amorphous silicon layer, which absorbs the incident light and separates when the energy of the absorbed light reaches a given value, is used as the separation layer and the silicon-based optical absorption layer. As the intervening layer for separating the two amorphous silicon layers, a silicon compound, for example, silicon oxide, is used.
(3) A method for transferring a transferred layer including a thin film device on a substrate onto a transfer member comprising: a first step for forming a separation layer on the substrate; a second step for forming the transferred layer including the thin film device on the separation layer; a third step for adhering the transferred layer including the thin film device to the transfer member with an adhesive layer; a fourth step for irradiating the separation layer with light through the substrate so as to cause exfoliation in the separation layer and/or at the interface; and a fifth step for detaching the substrate from the separation layer; wherein, in the fourth step, the stress, acting on the upper layers above the separation layer in the exfoliation in the separation layer and/or at the interface, is absorbed by the proof stress of the upper layers above the separation layer to prevent the deformation or breakage of the upper layers above the separation layer.
In the fourth step, the substances in the separation layer are optically or thermally excited by the incident light to cut bonds of atoms or molecules on the surface and in the interior and liberate the molecules and atoms to the exterior. This phenomenon is observed as phase transition, such as melting or evaporation, of the partial or entire substances in the separation layer. A stress acts on the upper layers above the separation layer as the molecules or atoms are released. The stress is, however, absorbed by the proof stress of the upper layers above the separation layer so as to prevent deformation or breakage of the upper layers above the separation layer.
The materials and/or thicknesses of the upper layers above the separation layer may be designed in view of such a proof stress. For example, one or more among the thickness of the adhesive layer, the thickness of the transferred layer, the material, and the thickness of the transfer member is designed in view of the proof stress.
Before performing the fourth step, the method further includes a step for forming a reinforcing layer for ensuring the proof stress at any position among the upper layers above the separation layer. In this invention, if the proof stress is not ensured only by the minimum configuration of the upper layers above the separation layer, consisting of the adhesive layer, the transferred layer, and the transfer member, deformation and breakage of the thin film device is prevented by adding the reinforcing layer.
(4) A method for transferring a transferred layer including a thin film device on a substrate onto a transfer member comprising: a first step for forming a separation layer on the substrate; a second step for forming the transferred layer including the thin film device on the separation layer; a third step for adhering the transferred layer including the thin film device to the transfer member with an adhesive layer; a fourth step for irradiating the separation layer with light through the substrate so as to cause exfoliation in the separation layer and/or at the interface; and a fifth step for detaching the substrate from the separation layer; wherein, the fourth step includes sequential scanning of beams for locally irradiating the separation layer, such that a region irradiated by the N-th beam (wherein N is an integer of 1 or more) does not overlap with other irradiated regions.
In the fourth step, beams, such as spot beams or line beams, for locally irradiating the separation layer are intermittently scanned so that substantially all the surface of the separation layer is irradiated with light. The beam scanning is achieved by relative movement between the substrate provided with the separation layer and the beam source or its optical system, and irradiation may be continued or discontinued during the relative movement. In this invention, the intermittent beam scanning is performed so that the adjacent beam-irradiated regions do not overlap with each other.
If the beam-irradiated regions overlap, the region may be irradiated with an excessive amount of light which will cause exfoliation in the separation layer or at the interface. It is clarified by analysis by the present inventor that an excessive amount of light partially leaks, is incident on the thin film device, and causes the deterioration of electrical characteristics and the like of the thin film device.
In the present invention, the separation layer is irradiated with such an excessive amount of light, hence the original characteristics of the thin film device are maintained after the thin film device is transferred onto the transfer member. A zone between individual beam-irradiated regions may be a low irradiation zone in which light is incident during the relative movement or a non-irradiation zone in which no light is incident during the relative movement. Exfoliation does not occur in the low irradiation zone or non-irradiation zone, the adhesion between the separation layer and the substrate can be remarkably reduced.
In the following two inventions, each beam for preventing or suppressing the characteristic deterioration of the thin film device is determined in different views from the invention in paragraph (4).
In the fourth step of the first invention, beams are sequentially scanned to irradiate locally the separation layer, each beam has a flat peak region having the maximum optical intensity in the center, and a region irradiated by the N-th beam (wherein N is an integer of 1 or more) does not overlap with other irradiated regions.
In the fourth step of the other invention, beams are sequentially scanned to irradiate locally the separation layer, each beam has the maximum optical intensity in the central region, and an effective region irradiated by the N-th beam (wherein N is an integer of 1 or more) having an intensity, which is 90% or more of the maximum intensity, does not overlap with the other effective regions irradiated by other beam scanning.
Since individual beams are scanned so that the flat peaks of individual beams or the effective regions having intensities which are 90% or more of the maximum intensity do not overlap with each other, two beams are continuously scanned in the same region in the separation layer.
The total irradiated beam (sum of the optical intensityxc3x97time) in the same region is lower than that when the flat peak region or the effective region having intensities which are 90% or more of the maximum intensity is set at the same position in the two consecutively scanned beams. As a result, the separation layer may separate after the second scanning in some regions, and this case does not correspond to the excessive irradiation. In another case, even if the separation layer separates in the first scanning, the intensity of the light incident on the thin film device in the second scanning is decreased, hence the deterioration of the electric characteristics of the thin film device can be prevented or reduced.
In the thin film device formed on a given substrate by a transfer technology of the thin film device (the thin film structure) in accordance with the present invention, the deterioration of various characteristics can be prevented or reduced by improving the irradiating step for exfoliating the separation layer.
When the thin film device is a thin film transistor (TFT), the improved irradiation step for exfoliating the separation layer can prevent the breakdown of the TFT due to a decreased on-current flow and an increased off-current flow in the channel layer of the TFT damaged by the incident light.
5. Further, the following inventions are disclosed in connection with the above-mentioned inventions.
A step for removing the separation layer adhered to the transfer member is provided for completely removing the unnecessary separation layer.
The transfer member is a transparent substrate. For example, inexpensive substrates such as a soda glass substrate and flexible transparent plastic films may be used as the transfer member. When the maximum temperature of the transfer member during the formation is Tmax, the transfer member is composed of a material having a glass transition point (Tg) or softening point which is lower than Tmax.
Although such inexpensive glass substrates etc. have not been used because they are not resistive to the maximum temperature of the conventional device production processes, they can be used in the present invention without restriction.
The glass transition point (Tg) or softening point of the transfer member is lower than the maximum temperature in the process for forming the thin film device. The upper limit of the glass transition point (Tg) or softening point is defined. The transfer member is composed of a synthetic resin or a glass material. For example, when the thin film device is transferred onto a flexible synthetic resin plate such as a plastic film, excellent characteristics which are not obtainable in a glass substrate with high rigidity can be achieved. When the present invention is applied to a liquid crystal device, a flexible lightweight display device which is resistive to falling can be achieved.
Also, a thin film integrated circuit such as a single-chip microcomputer including TFTs can be formed by transferring the TFTs on a synthetic resin substrate by the above-mentioned transferring method.
An inexpensive substrate such as a soda-glass substrate can also be used as the transfer member. A soda-glass substrate is inexpensive and thus has economical advantages. Since alkaline components are dissolved from the soda-glass substrate during annealing of the TFT production, it has been difficult to apply active matrix liquid crystal display devices. In accordance with the present invention, however, since a completed thin film device is transferred, the above-mentioned problems caused by the annealing will not occur. Accordingly, substrates having problems in the prior art technologies, such as a soda-glass substrate, can be used in the field of active matrix liquid crystal display devices.
The substrate has thermal resistivity: The thin film device can be annealed at a high temperature in the production process, and the resulting thin film device has high reliability and high performance.
The substrate has a transmittance of 10% or more for the 310 nm light: The transparent substrate can supply optical energy sufficient to ablation in the separation layer.
When the maximum temperature in the formation of the transferred layer is Tmax, the substrate is composed of a material having a distortion point of Tmax or more: The thin film device can be treated at a high temperature in the production process, and the resulting thin film device has high reliability and high performance.
The separation layer may be composed of amorphous silicon: The amorphous silicon can absorb light, can be easily produced, and has a highly practical use.
The amorphous silicon contains 2 atomic percent or more of hydrogen (H): When the amorphous silicon containing hydrogen is used, hydrogen is released by light irradiation, and an internal pressure occurs in the separation layer to promote exfoliation in the separation layer. The amorphous silicon may contain 10 atomic percent or more of hydrogen (H). The exfoliation in the separation layer is further accelerated by the increased hydrogen content.
Alternatively, the separation layer may be composed of silicon nitride: When using silicon nitride as a separation layer, nitrogen is released by light irradiation to promote exfoliation in the separation layer.
Alternatively, the separation layer may be composed of a hydrogen-containing alloy: When using a hydrogen-containing alloy, hydrogen is released by light irradiation to promote exfoliation in the separation layer.
Alternatively, the separation layer may be composed of a nitrogen-containing alloy: When using a nitrogen-containing alloy, nitrogen is released by light irradiation to promote exfoliation in the separation layer.
The separation layer may be composed of a multi-layered film: The separation layer is therefore not limited to a single-layered film. The multi-layered film is composed of an amorphous silicon film and a metallic film formed thereon.
The separation layer may be composed of at least one material selected from the group consisting of ceramics, metals, and organic polymers. Usable metals include, for example, hydrogen containing alloys and nitrogen containing alloys. As in amorphous silicon, exfoliation in the separation layer is accelerated by the evolution of gaseous hydrogen or nitrogen by light irradiation.
The light is laser light. Laser light is coherent light and is suitable for causing exfoliation in the separation layer. The laser light has a wavelength of 100 nm to 350 nm. The short-wave, high energy laser light results in effective exfoliation in the separation layer. An example of such a laser is an excimer laser. The excimer laser is a gas laser which is capable of outputting laser light with high energy, and four typical types of laser light can be output (XeF=351 nm, XeCl=308 nm, KrF=248 nm, ArF=193 nm) by combinations of rare gasses (Ar, Kr, and Xe) and halogen gasses (F2 and HCl) as laser media. By excimer laser irradiation, direct scission of molecular adheres and gas evolution will occur in the separation layer provided on the substrate, without thermal effects.
The laser light may have a wavelength of 350 nm to 1,200 nm. For the purpose of imparting exfoliation characteristics to the separation layer by changes, such as gas evolution, vaporization, and sublimation, laser light having a wavelength of 350 nm to 1,200 nm can also be used.
The thin film device may be a thin film transistor (TFT). The TFT may be a CMOS-type TFT.
A high-performance TFT can be transferred (formed) on a given transfer member without restriction. Various electronic circuits can therefore be mounted on the transfer member. Accordingly, by the above-mentioned inventions, a thin film integrated circuit device including the thin film device transferred onto the transfer member is achieved. Also, a liquid crystal display device including an active matrix substrate, which is produced by the transfer of the thin film transistors in the pixel region, is achieved, wherein the pixel region includes a matrix of thin film transistors and pixel electrodes each connected to one end of each thin film transistor.